The present invention relates to a scanning optical system configured to simultaneously deflect a plurality of light beams and to converge the plurality of beams on a scan target surface.
Such a scanning optical system is referred to as a multi-beam scanning optical system. Since the multi-beam scanning optical system forms a plurality of scan lines on the scan target surface by simultaneously deflecting the plurality of beams on one reflective surface of a deflector (e.g., a polygonal mirror), imaging speed (i.e., printing speed) can be enhanced.
In this specification, a direction in which a beam spot is scanned on the scan target surface is referred to as a main scanning direction, and a direction perpendicular to the main scanning direction on the scan target surface is referred to as an auxiliary scanning direction.
Japanese Provisional Publication No. SHO 57-54914 (document 1) discloses a multi-beam scanning optical system in which a single light emitting device having a plurality of light emitting points is employed.
Japanese Provisional Publication No. SHO 61-15119 (document 2) discloses another type of the multi-beam scanning optical system in which a plurality of light emitting devices each of which has a single light emitting point are provided.
In both cases of the scanning optical systems disclosed in the document 1 and the document 2, space between adjacent light emitting points can not be shortened under a certain limit determined by a dimensional requirement of the light emitting device. Therefore, if a multi-beam scanning optical system is configured such that the light emitting points are arranged along a line parallel with the auxiliary scanning direction by using the conventional light emitting device, imaging quality is deteriorated. The reason is that a plurality of scan lines formed on a scan target surface by the beams passing through a line image forming lens and an imaging optical system deviate from each other without overlapping with respect to each other.
For this reason, several techniques to make the scan lines overlap one another have been proposed. A first technique is to increase diameters of beam spots on the scan target surface by locating an aperture stop for each beam at a pupil position of the line image forming lens.
However, the first technique has a problem that considerable part of light energy of the beam emitted from each light emitting point is lost by the aperture stop.
A second technique is to reduce intervals of the beam spots in the auxiliary scanning direction by inclining a direction of alignment of the plurality of light emitting points with respect to the auxiliary scanning direction.
Although the second technique enables to reduce intervals of the scan lines, it raises a problem that a direction of the major axis of each beam of a semiconductor laser in far field (i.e., a direction of the major axis of the beam spot on the scan target surface) becomes substantially perpendicular to the auxiliary scanning direction. To avoid this phenomenon, it is required to employ an anamorphic optical system or an aperture stop having the form of a slit for directing the major axis of the beam spot to be parallel with the auxiliary scanning direction.
To employ the anamorphic optical system in the multi-beam scanning optical system increases manufacturing cost of the multi-beam scanning optical system. To employ the aperture stop having the form of the slit reduces efficiency of use of light power of the laser beams.
Even though a diameter of each beam spot can be increased by increasing a lateral magnification of the whole scanning optical system, an increase of the lateral magnification also increases the intervals of the beam spots. Accordingly, in this case, to make the scan lines overlap one another is impossible.